Because of their large-capacity and high-density memory, optical information recording media have been receiving attention. The development of an erasable type from which information can be erased and on which information can be rewritten has been pursued at present. The erasable-type optical information recording media include one in which thin films whose phase is changed between an amorphous state and a crystalline state are used as recording layers and information is recorded and erased by thermal energy through the irradiation of laser beams.
As a phase-change material for the recording layers, an alloy film containing Ge, Sb, Te, In, or the like as the main component, for example, a Ge—Sb—Te alloy film, has been known. In many cases, information is recorded by formation of marks through partial change in a recording layer into the amorphous state and information is erased by crystallization of the amorphous marks. The amorphous state can be obtained by heating the recording layer to its melting point or more and then cooling it at a speed of at least a fixed value. On the other hand, the crystallization can be carried out by heating of the recording layer to a temperature between its crystallization temperature and its melting point.
Recently, with the improvement in processing capability of various types of information-processing equipment, the amount of information to be processed has been increasing. Therefore, large-capacity recording media on which information can be recorded or from which information can be reproduced at high speed have been demanded. As a means for increasing the capacity and speed, for example, JP-A-3-157816 describes a recording medium having a plurality of recording layers and transparent separating layers provided between the recording layers and a recording/reproducing apparatus for the same. In the first recording/reproducing apparatus described in the above-mentioned JP-A-3-157816, light is focused on an intended recording layer out of the plurality of recording layers in the medium by inserting a parallel plate whose thickness is varied depending on the intended recording layer, between an objective lens and a medium. The second recording/reproducing apparatus in JP-A-3-157816 comprises one objective lens and a plurality of light sources that emit lights with different wavelengths. By utilizing the chromatic aberration of the objective lens, focal positions of respective lights irradiated from the plurality of light sources onto a medium are varied from one another. As a result, the lights are focused on a plurality of recording layers respectively at the same time. Further, in the third recording/reproducing apparatus in JP-A-3-157816, a plurality of light sources that emit lights with the same wavelength are positioned so as to have different position relationships from one another with respect to an optical system and thus the focal positions of respective lights irradiated from the plurality of light sources onto a medium are varied from one another. Consequently, the lights are focused on a plurality of recording layers respectively at the same time.
In the above-mentioned first recording/reproducing apparatus, the demand for the increase in capacity can be satisfied. However, the other demand for the increase in speed has not been satisfied sufficiently since information cannot be reproduced from or recorded in a plurality of recording layers simultaneously and therefore the data rate has been the same as in the case of using a medium with a single recording layer. Further, in the second and third recording/reproducing apparatuses, a common shifting of the objective lens is used for focusing lights irradiated from the plurality of light sources. Therefore, the relative relationships among the focal positions of lights irradiated from respective light sources are continuously constant. Consequently, when the thickness of transparent separating layers in the recording medium is different from a designed value or is uneven, it was difficult to focus the lights on the plurality of recording layers precisely, which has been a problem.